(1) Field of the Invention
This invention relates to a heat pipe, in particular, a grooved flat-plate heat pipe used for heat dissipation for electronic integrated circuit (IC) chips, such as a central processing unit (CPU), or other high-flux heat sources.
(2) Brief Description of Related Art
The latest generation of Pentium IV CPU generates power more than 100 watts (Joule/sec). In order to maintain its normal performance and avoid overheating of the unit, more effective heat dissipating mechanism is needed. A method uses a flat-plate heat pipe, as shown in FIG. 1, for spreading the heat generated by a semiconductor device. An enclosed and vacuumed metal chamber 11, whose inner surface covered with a layer of wick 10, contains a working liquid undergoing a cycling loop. The wick material is commonly sintered metal powder, metal wire mesh, or metal cloth, etc. A part of the wick on the lower surface serves as the evaporator, where the heat from the attached heat generating device is absorbed by the evaporation of the working liquid. The working vapor spreads over the internal space of the enclosed chamber 11 to make it nearly isothermal. The vapor condenses on the top surface where a heat sink (not shown) is connected. The condensed liquid flows back to the evaporator via the wick, as shown in arrows in FIG. 1. There are at least two disadvantages for such a scheme. First, as the wick is porous, large friction tends to retard the liquid back flow and lead to dry-out at high heat loading. Second, as the vacuumed chamber faces with inward pressure, deformation is avoided either by inserting stiffeners or by locally squeezing the chamber to cause local contacts of the top and bottom layers (not shown).
To improve the first disadvantage, the porous wick 10 can be replaced with the parallel grooves 9, as shown in FIG. 2, to reduce the friction for liquid flow. Again, stiffeners are needed to withstand the inward pressure.
To improve the second disadvantage, U.S. Pat. No. 3,613,778 disclosed a structural wick 6a to withstand the inward pressure, as shown in FIG. 3. Two layers of porous wick 12 and 13 with parallel grooves 14 are disposed face to face within the enclosed metal chamber 11. The groove orientations of each wick layer are perpendicular. The space composed by the perpendicular grooves 14 serves as the vapor conduit, while the liquid conduit is the interior pores of the porous wick. The first disadvantage of large friction for liquid flow, however, is not improved in this embodiment.
U.S. Pat. No. 6,679,318 disclosed another single-layer, waffle-shaped structural wick made of sintered metal powders, to withstand the inward pressure. It also disclosed an anti-pressure layout with the upper and lower sections, both containing porous non-structural layers. The small pores within the lower-section wick layer serve as the liquid conduit, while the large pores within the upper-section metal wire mesh or metal open cell foam sheet serve as the vapor conduit.
U.S. Patent Application 2005-0183847-Al disclosed a flat-plate heat pipe as shown in FIG. 4. The interior of the enclosed metal chamber 100 comprises the upper and lower sections designed for coolant cycling. The route for the working fluid is shown therein. The upper section includes a set of parallel V-shaped grooves 201 lying over the wick layer 203 in the lower section. Between the parallel grooves 201 and the wick layer 203 is disposed another dividing layer 205 made of permeable metal or nonmetal material. An open space 300 is thus retained for lateral vapor motion. When the heat pipe is heated with a CPU or other heat generating device, the working liquid (not shown) in the wick 203 evaporates. The vapor spreads over the upper section and condenses on the walls 202 of the parallel grooves 201, with the condensation heat dissipated out through the heat sink (not shown) connected with the upper chamber wall. The condensed liquid passes through the permeable dividing layer 205 to directly, without flowing through a long route, enter the wick layer 203 in the lower section. The evaporation of the liquid in the wick 203 leads to a liquid-vapor interface within the wick 203. This liquid-vapor interface results in a capillary pulling force on the working liquid on the groove walls 202 and in the wick 203 toward the evaporation region to make a full cycle: liquid→vapor→cooling→liquid following the arrows as shown in FIG. 4. However, when single or multiple layers of metal mesh or metal cloth is adopted as the wick 203, the wick tends to bump up without some kind of strengthener between the top wall and the wick 203 to press it down. This leads to attenuation of capillary force for the back flow of the working liquid and the subsequent dry-out.
The differences between this invention and the embodiment of U.S. Pat. No. 3,613,778 include: (1) The present groove structure is made of non-porous materials, preferably fabricated on the top chamber wall 101 as a unitary cover. In contrast, the embodiment of U.S. Pat. No. 3,613,778 adopts a structural porous wick, which can not be fabricated on the metal wall as a unit; (2) The present porous wick in the lower section is a simple non-structural wick layer, rather than a structural wick; (3) The vapor conduit in this invention is the upper-section channel space connected by the cut-off openings, in contrast with the cross-linked space of the upper and lower grooves; (4) The liquid conduit in this invention includes upper-section groove wall and groove corners and the lower-section wick pores, in comparison with the wick pores in both structural wick layers. The superiorities of this invention include: (1) When the grooves are fabricated on the top wall as a unit, the structure is simpler with better heat transfer characteristics because no contact thermal resistance exists between the grooves and the top chamber wall. In contrast the sintered-metal-powdered structural wick cannot be fabricated with the top wall as a unit. Contact thermal resistance exists between the wick and the top wall and retards the heat transfer across the top wall; (2) The friction of liquid flow associated with the groove corners and groove walls is much smaller than that associated with the fine pores within the wick, and (3) Both the upper-section groove structure and the lower plain wire-mesh wick layer in the are simpler and cheaper than the structural wick with grooves.
The differences between this invention and the embodiment of U.S. Pat. No. 6,679,318 include: (1) The present upper-section structure is a grooved structure made of non-porous materials, preferably fabricated on the top chamber wall 101 as a unitary cover. In contrast, the embodiments of U.S. Pat. No. 6,679,318 adopt porous wick made of either metal wire mesh or open cell foam sheet, which can not be fabricated on the metal wall as a unit; (2) The vapor conduit in this invention is the upper-section smooth channel space connected by the cut-off openings, in contrast with the internal non-smooth pores of the metal wire mesh or the open cell foam; (3) The liquid conduit in the upper section in this invention includes smooth groove wall and groove corners, in comparison with the non-smooth walls of the metal wire mesh or the open cell foam. The superiorities of this invention are: (1) When the grooves are fabricated on the top wall as a unit, the structure is simpler with better heat transfer characteristics because no contact thermal resistance exists between the grooves and the top chamber wall. In contrast, none of the waffle-shaped wick, metal wire mesh, or metal open cell foam can be fabricated with the top wall as a unit. Contact thermal resistance exists between the wick and the top wall and retards the heat transfer across the top wall; (2) The friction for vapor flow associated with the smooth groove walls is much smaller than that associated with the non-smooth. pores within the metal wire mesh or metal open cell foam; (3) The friction for liquid flow in the upper section associated with the smooth groove walls and groove corners is much smaller than that associated with the non-smooth surface of the metal wire mesh or metal open cell foam. Besides, a large number of corners exist at the intersecting locations of the mesh wires or the irregular foam surfaces. Their remarkable capillary force attracts and holds a considerable amount of working liquid. This tends to retard liquid cycling.